The use of pedicle screws is commonly known in surgery. As a rule, they are used in combination with a rod in order to fix vertebral bodies relative to one another, for example. In doing so, the pedicle screws are turned into vertebral bodies which are to be fixed relative to one another and subsequently are connected to one another usually with the aid of a rod. To this end, the head of the pedicle screw, which is also referred to as a tulip, has a recess which is U-shaped in most cases. For the mutual fixation of bones or bone pieces, the rod is inserted in the tulip and clamped in place therein with the aid of a screw fastening, so that the pedicle screws and hence the bones or bone pieces which have the screws driven into them are fixed relative to one another via the rod. The process of clamping the rod is usually carried out by use of grub screws, so-called set screws.
It may happen under certain circumstances that the rod prior to clamping it in place is not perfectly received in the tulip or only partially received therein. In such a case, it has to be correctly positioned in the tulip first; only then it can be clamped in the tulip in said position by screwing in the set screw. In order to position the rod in the recess of the pedicle screw, it is pressed into the tulip with an instrument which is referred to as a rod pusher or rod rocker or the pedicle screw is pulled up to it.
It is unavoidable that the rod which is pushed by force into the tulip with the aid of such an instrument is under mechanical tension in most cases. In this case, it is required to screw in the set screw into the tulip at least partially, while the rod is retained with the aid of the rod pusher, so that it remains in the dedicated recess of the tulip. The process of screwing in the set screw is usually performed with a screwdriver, while care has to be taken here that the pedicle screw does not turn during screwing in the set screw. Depending on the design of the implant, it is advantageous or even mandatory to introduce the set screw exactly in the direction of the thread axis, which may be problematic with certain operation conditions. Bearing this in mind, the prior art knows to provide a guiding arrangement in the implant for guiding the set screw and/or the screwdriver. It is also known to employ an auxiliary guiding instrument in addition to the rod pusher. In order to prevent the pedicle screw from turning upon driving in the set screw, it is known to use counter torque devices with which the pedicle screw is manually held by the surgeon or other surgical staff.
For positioning a surgical rod in an implant, the prior art knows various instruments. One example is a lever-type instrument, the one side of which comprises a handle and the opposite side of which comprises a fork-type instrument head. The instrument head can be pivotally arranged on the head of an implant, e.g. a pedicle screw, and comprises a lever-type spaced contact surface for the surgical rod. By swiveling the instrument around the head of the implant, the contact surface for the surgical rod is moved toward the rod seating of the pedicle screw. The rod can then be held in the desired position with the aid of the instrument. A disadvantage is that the arrangement and positioning of the instrument on the implant is afflicted with a certain instability, so that it may come loose in some circumstances. Furthermore, the instrument exclusively fulfils the function of positioning and holding the rod in the rod seating of the pedicle screw and to this end has to be manually held in the required position. It is disadvantageous that it does not offer any guidance which assists the surgeon during screwing in the set screw.
Another example for a known instrument is a pair of pliers whose implant-side pliers arms are formed into a seating for the head of a pedicle screw. The pair of pliers can be opened by the surgeon, can be arranged on the pedicle screw head so as to encompass it and can be closed subsequently, so that the seating of the pair of pliers is safely arranged on the screw head. The pair of pliers comprises a ratchet by means of which it remains in the closed position and it cannot be removed from the pedicle screw without releasing the ratchet. The plier arms have a lever-type design and comprise a contact surface for the surgical rod between them in the closed state, so that the latter can be inserted in a dedicated seating of the tulip by swiveling the pair of pliers around the pedicle screw head. It is easy to arrange the pair of pliers on the pedicle screw head, but a permanently reliable fit on the pedicle screw head requires a correct positioning and the application of a sufficient closing pressure. It is also necessary that the pair of pliers is manually held in the position which is required for positioning and clamping the surgical rod. It is disadvantageous that the instrument does not offer any guidance which assists the surgeon during screwing in the set screw.
Another known device comprises axially aligned, mutually concentric sleeves which can be moved relative to each other in axial direction by means of a lever ratchet mechanism. An inner sleeve is arranged on the head of a pedicle screw. An outer sleeve which is movable with respect to the inner sleeve comprises a seating for a surgical rod or forms such a seating. An actuation of the lever mechanism makes the outer sleeve together with the surgical rod move toward the inner sleeve which is arranged on the pedicle screw head, so that the rod is positioned relative to the screw head in the seating thereof. The inner spaces of the inner and outer sleeve simultaneously form a guide for a screw-driving tool for screwing in a set screw which is always aligned with the pedicle screw head.
Finally the patent document U.S. Pat. No. 8,449,549 B2 discloses in several embodiments an instrument realized as a sort of pliers which is termed as “Rod Coercer”. It comprises two plier branches which are arranged on each other so as to be rotatable around a first hinge and comprise handles for a user-side actuation. The pedicle-screw side ends of the pair of pliers opposite to the handle side together form a contact surface for a surgical rod in the closed state of the pair of pliers. Each of the pedicle-screw side ends have an implant gripper arm arranged thereon so as to be able to swivel around a second hinge whose swivel axis is orthogonal to that of the first hinge. Upon opening the instrument, its implant gripper arms and its pedicle-screw side ends will open as well. During closing the instrument, its implant gripper arms and its pedicle-screw side ends will be closed, too. The instrument is applied on a pedicle screw in the opened state and is subsequently closed. In doing so, the implant gripper arms are supposed to enclose the pedicle screw head and the pedicle-screw side ends should rest against each other to a more or less defined extent and define the contact surface for the surgical rod. The pedicle-screw side ends are freely movable relative to each other in two directions in space due to the two swivel axes of the hinge being orthogonal to each other. As a consequence, the right working end may have another position than the left working end during use of the instrument, and the two working ends may get tilted relative to each other. Thus, the instrument does not provide the surgeon with a direct feeling and feedback with respect to the situation on the pedicle screw head. The handling of the instrument and in particular its positioning relative to the pedicle screw head and the surgical rod are difficult. The instrument does not comprise any guiding means for the set screw or for a tool for screwing in the set screw. The function of the instrument as a counter-torque device, in order to prevent the pedicle screw from being turned during screwing in the set screw, disadvantageously depends on the correct positioning on the pedicle screw head and a sufficiently high closing pressure of the instrument.